A mechanical transmission, tethered actuation system, an autonomous ankle exoskeleton design and method of their use employing a cable, pulleys and associated pulley housings to change angular transmission of linear force on the cable. The pulleys are linked by a ground link and the cable is threaded across and between the pulleys, whereby rotation of either of the pulleys in one direction causes rotation of the other pulley in the opposite direction. Independently of the pulleys, the pulley housings can freely rotate about associated pulleys, and a link between the pulley housings is provided, whereby rotation of one of the pulley housings in one direction causes rotation of the other pulley housing at an equivalent angle in the opposite direction, thereby enabling a change in transmission angle of linear force on the cable threaded across and between the pulleys and the associated pulley housing essentially without resistance. When pulleys have the same angular velocity ratio as that of the associated pulley housings, there is no cable slack since the net changes in length of the cable wrapping around two pulleys is zero.
